He's right to say organic food has 'significant benefits in terms of animal welfare and reducing fuel costs'. But Gray says our only choices in future are GM crops or industrial-style agriculture and that, on its own, organic farming will do nothing to stop 'the devastation of wilderness' necessary to feed a growing population. He ignores three facts.

First, industrial farming is devastating rain forests to provide protein for industrial livestock production and plant oils for industrial biofuel. Second, his options - GM and industrial farming - rely on oil or other fossil fuels to provide the high levels of fertility they need (via artificial nitrogen fertiliser made with oil or gas). Third, he misrepresents the alternative. We need to move to much more seasonal diets, with organic food, sourced locally whenever possible, and with fewer meat and dairy products.

Organic farming could feed the world now, taking the Sun's energy, using clover to turn that into nitrogen, and thus to food. That is the future.Peter MelchettPolicy Director, Soil Association, Bristol

John Gray's piece was a brilliant article by the most important British political thinker of our time. The perverse fear of nuclear power is based on nightmares dating from the Cold War, the nuclear arms race and CND.

Nuclear power is by far the cleanest and one of the safest ways of producing energy. The number of people who have died from radiation is minuscule compared with those who will be killed through global warming. Karl Szekelyvia email

Where did Mr Gray form the opinion that 'green activists' are against population control? The mainstream recognises that the fundamental aim of reducing human impact on the planet cannot ignore the fundamental problem of human population.

Key figures in the environmental movement discussed precisely those subjects as long ago as the Sixties. The debate continues.

Nowadays, education of women and development are considered to be much more successful methods of reducing fertility than any method involving compulsion.Dean Morrisonvia email

I welcome John Gray's thoughtful article. The risks are so great that countries, business and people should be not just curbing their greenhouse gas emissions, as he argues, but seeking to eliminate them altogether.

He is right to argue that all mitigation options should be considered, with nuclear, despite its long-lived toxic wastes, as one of them, although he omits to mention explicitly the importance of carbon capture and storage. In fact, the challenge of weather forecasters, engineers and planners is to find, exploit and share the variety of low-cost options available. These are to be found in the supply and demand for energy. There are many different natural sources of energy - sun, wind, rainfall, tides, and human beings - across countries and human activities.

However, he is wrong to suggest that economic expansion or population growth must be slowed or stopped. This is an extreme view: the contribution of fossil fuels to the world economy is remarkably small, probably less than 4 per cent of GDP. The known options are price competitive and some, such as nuclear and solar, are abundant.Dr Terry BarkerCambridge Centre for Climate Change Mitigation Research

Being an only child and having one child, being a veggie, making small cars last 20 years, can I consider myself a good citizen of the planet?Bill HiggsLadysmith, Canada

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